perf tools: Add 'make DEBUG=1' to remove the -O6 cflag

When using gdb to debug perf, it is practically impossible to
use when perf is compiled with -O6. For developers, this patch
adds the DEBUG feature to the make command line so that a
developer can easily remove the optimization flag.

LKML-Reference: <1255590330.8392.446.camel@twins>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <20091020232033.984323261@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>